When you know you don’t want the job, informing the interviewer is a professional courtesy. "No further interest" letters are difficult, but they are an opportunity to turn an awkward situation into a strengthened relationship. Your objective for this letter is to maintain the respect and confidence of someone who thought enough of you to bring you in for a meeting or final interview – and someone who might know of a job that does interest you in the future. 

Elements of a "no further interest" letter:

• Appreciation
  Thank them for their time.
  
  
• Decline
  Statement of why the job is not a good fit for you.
  
  
• Next Step
  Solicit feedback and state you will contact them at some future date.

View no further interest letter example